The goal of the proposed project shall be to gain quantitative information on the number, nature, and acute radiation response of the cells involved in initiation of the neoplastic process which leads to mammary tumors following exposure of mammary cells to ionizing radiation in vitro, and how this process may be related to the type of radiation, the dose, and the hormonal exposure of the cells before and after exposure. The project shall be divided into short-term studies devoted to development and application of a radiation dose-mammary cell survival assay, and to long term carcinogenesis experiments. The former will be based on grafting of monodispersed mammary cell suspensions into the mammary areas of mammary gland-free rats which are primed with injected hormones and/or grafted with mammotropin-secreting pituitary tumors. Long-term studies will involve irradiation of mammary cell suspensions in vitro which will then be grafted in similar animals which will be observed for tumor development. By these methods we hope to a) determine the total number of cells of the type from which the malignancy is derived and which survive exposure, b) minimize the abscopal effects of irradiation, and c) develop a system in which both physical and hormonal factors can be manipulated before and after exposure for study of cell survival, repair and neoplasia in a quantitative fashion.